The present invention relates to the transfer of thermal energy to and from subsurface and submarine environments to be utilized by any heating/cooling system as well as any power generating system that could exploit a temperature differential within a moving fluid.
Increasing awareness of the limited supply of fossil fuel reserves has raised interest in alternative energy sources. Cost conscious consumers have expressed interest in solar power and wind power alternative energy sources to supplement or replace conventional fossil-fuel based systems. Extensive research in these two fields and the fact that they are able to supplement an existing electrical infrastructure has focused much interest in these two areas. However, the limitations of weather-dependent solar and wind technologies are apparent. They are, in effect, interruptible electrical power suppliers.
Geothermal, a third source of energy, can provide a much more reliable source of alternative, renewable, non-polluting energy in a thermal form. The field of geothermal energy encompasses two substantially different disciplines. The first, involves the extraction of thermal energy in the form of steam from below-ground sources near (volcanic) magmatic regions. Water pumped into overlying ground fissures quickly absorbs abundant quantities of heat and is transformed to steam which is channeled to perform useful work. When the term “geothermal energy” is used, this method is often the one that is being described. However, the same term is often used to describe the second discipline, which is also the subject of the present invention. All references to “geothermal energy” in the description of the current invention will be based on the second discipline, which is described next. The second discipline is the transfer of thermal energy to and from relatively shallow depths below the surface of the Earth using a liquid thermal transfer medium and at temperatures generally much lower than that of steam. This methodology is often referred to as ground-source heat transfer, earth-coupled, geothermal heat pump and GeoExchange systems. A subsurface closed-loop system in which a finite amount of thermal transfer fluid is re-circulated to transfer heat between the earth and an above ground heating/cooling load is the most common ground source heat transfer system in use today although there are other open-loop type systems. Underground loops commonly fabricated from copper or polyethylene transport thermal transfer fluid, consisting of a refrigerant or aqueous solution, respectively, to an indoor facility to accomplish heating and/or cooling. Loops are placed in a predominantly vertical or horizontal orientation. Installation of vertical loop systems necessitates the use of bulky, expensive drilling rigs to drill one or more boreholes that are approximately four inches in diameter and two to four hundred feet deep. Within this deep narrow borehole a U-shape length of tubing is inserted to extract thermal energy from, or to put into, the ground by circulating a thermal transfer fluid within it. Horizontal loop systems require a substantially large surface area under which trenches approximately three or more feet in width and four to eight feet deep need to be excavated to install the loops in a variety of configurations. Loops fabricated from polyethylene, meant to carry an aqueous solution, must be considerably longer than their copper/refrigerant counterpart because the fluid they carry is at a much lower temperature differential relative to the surrounding earth and their thermal conductivity is much lower than that of copper. In addition, for both copper and polyethylene loops, the surrounding earth is susceptible to being depleted of thermal energy (in the heating mode) and saturated with thermal energy (in the cooling mode). In order for the average near-loop earth temperature to be constant, and therefore an effective heat-sink, a loop of sufficient length is required to mitigate the depletion/saturation problem. This also mandates a minimum separation between loops. The near-loop depletion/saturation of thermal energy occurs because the heat capacity of soil, its ability to absorb a large amount of heat with only a small change in temperature, is not as high as some other materials. In addition, this property, known as specific heat, varies based on soil composition, compaction and moisture content. The most effective material to surround the loops for heat transfer would be one that could absorb/release large amounts of thermal energy with the smallest change in temperature.
Based on the above considerations, there are two major impediments to the widespread implementation of ground-source geothermal systems, particularly for smaller residential applications. First, the installation of vertical, deep, borehole ground loop systems requires the use of expensive borehole drilling equipment with costly up-front capital expenditures resulting in a long payback period. Second, horizontal loop systems require a relatively large land surface area to be effective, severely limiting the pool of potential users. Their installation also has the potential to damage surface embellishment such as lawns and shrubbery, another discouraging factor.
Consequently, a ground-source geothermal system that could mitigate the above impediments would be desirable from both a cost and aesthetic perspective. A hybrid system that employed multiple, shallow, vertical boreholes, on the order of approximately twelve to twenty five feet deep and two to four feet in diameter, capable of being drilled by less expensive equipment than the large drilling rigs, would lower initial capital costs significantly. Such equipment is commonly used today to drill holes for utility and telephone pole installation. In order for the shallower borehole system to be effective, based on the above considerations, three implementation characteristics are necessary. They are:    1. The loop within the borehole cannot be the simple U-shape configuration with straight vertical segments; currently the standard method used for deep borehole geothermal systems. A method of increasing the effective loop contact area with the surrounding earth in a shallower borehole must be employed.    2. The loop must be surrounded by, and in thermal contact with, material of much higher heat retention characteristics than that of common soil. The heat retention characteristics of soil can vary widely depending on compaction; on composition, such as clay, sand or stone; and to a much greater degree based on moisture content. By surrounding the loop(s) with material of high, constant, heat retention characteristics, the efficacy and efficiency of the system is enhanced and made resistant to thermal fluctuation.    3. The system must be able to be assembled and installed with a minimum of expensive equipment and personnel costs.
Prior art geothermal disclosures have attempted to improve on requirement (1) above. U.S. Pat. No. 5,623,986 discloses a heat exchange system with a helically wound loop. A major disadvantage of this implementation is that only one-half of the subsurface loop is helically wound while the other uncoiled half is entirely insulated to eliminate intra-loop thermal “short-circuiting” between the two legs of the U-shape loop. The result is that only one-half of the U-loop is useful in heat transfer, and the additional expense of insulation material and its installation is incurred.
U.S. Pat. No. 5,054,541 discloses a ground coil assembly geothermal system designed for shallow depths. The design satisfies requirement (1) above with a helically wound loop which insures a large thermal transfer capability with the surrounding earth. A disadvantage is the possibility of thermal “short-circuiting” between sections of the loop, which are in close proximity. Because the entire unit is prefabricated and therefore must be transported to the installation site, there is a practical limitation on the borehole depth that can be used.